Method of adhering wire bond loops to reduce loop height

ABSTRACT

A method of reducing wire bond loop heights in wire bonds electrically connecting an integrated circuit die with a contact pad to a printed circuit board with a conductor, by mounting the integrated circuit die such that the contact pad is spaced from the conductor, positioning an adhesive surface between the contact pad and the conductor on the printed circuit board, attaching wire to one of the contact pad or the conductor, drawing the wire towards the other of the contact pad or the conductor, allowing the wire to contact the adhesive surface, and, attaching the wire to the other of the contact pad of the conductor to form a wire bond adhered to the adhesive surface and a point intermediate its ends.

CROSS REFERENCES TO RELATED APPLICATION

The present application is a Continuation-In-Part of U.S. Ser. No.11/860,539 (Our Docket MPN008US), filed Sep. 25, 2007, the contents ofwhich are incorporated herein by cross reference.

FIELD OF THE INVENTION

The invention relates to the field of integrated circuits. Inparticular, the invention is directed to the wire bonds between acircuit board and the contact pads on the integrated circuit die.

CO-PENDING APPLICATIONS

The following application has been filed by the Applicant simultaneouslywith the present application:

-   -   MPN015US MPN016US MPN017US

The disclosure of this co-pending application is incorporated herein byreference. The above application has been identified by its filingdocket number, which will be substituted with the correspondingapplication number, once assigned.

CROSS REFERENCES

Various methods, systems and apparatus relating to the present inventionare disclosed in the following US Patents/Patent Applications filed bythe applicant or assignee of the present invention.

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7144519 6799835 69599756959974 7021740 6935718 6938983 6938991 7226145 7140719 6988788 70222506929350 7011393 7004566 7175097 6948799 7143944 7310157 7029100 69578117073724 7055933 7077490 7055940 10/991402 7234645 7032999 70665767229150 7086728 7246879 7284825 7140718 7284817 7144098 7044577 72848247284827 7189334 7055935 7152860 11/203188 11/203173 7334868 721398911/225156 11/225173 7300141 7114868 7168796 7159967 7328966 715280511/298530 11/330061 7133799 11/330054 11/329284 7152956 7128399 71473057287702 7325904 7246884 7152960 11/442125 11/454901 11/442134 11/45044111/474274 11/499741 7270399 6857728 6857729 6857730 6989292 71262166977189 6982189 7173332 7026176 6979599 6812062 6886751 10/80405710/804036 7001793 6866369 6946743 7322675 6886918 7059720 730630510/846562 7334855 10/846649 10/846627 6951390 6981765 6789881 68025927029097 6799836 7048352 7182267 7025279 6857571 6817539 6830198 69927917038809 6980323 7148992 7139091 6947173 7101034 6969144 6942319 68274276984021 6984022 6869167 6918542 7007852 6899420 6918665 6997625 69888406984080 6845978 6848687 6840512 6863365 7204582 6921150 7128396 69133477008819 6935736 6991317 7284836 7055947 7093928 7100834 7270396 71870867290856 7032825 7086721 7159968 7010456 7147307 7111925 11/1448127229154 11/505849 11/520570 7328994 7341672 11/540575 11/583937 72787117290720 7314266 11/635489 11/604319 11/635490 11/635525 728770611/706366 11/706310 11/706308 11/785108 11/744214 11744218 1174848511/748490 11/764778 11/766025 11/834635 11839541 11860420 11/86569311/863118 11/866307 11/866340 11/869684 11/869722 11/869694 11/87659211/945244 11951121 11/945238 11955358 11965710 11962050 1201547812015423 12015434 12023015 12030755 12025641 12036279 12031598

BACKGROUND OF THE INVENTION

An integrated circuit fabricated on the silicon wafer is often referredto as a ‘die’. For the purposes of this specification, the term die willbe used as a reference to an integrated circuit fabricated on a wafersubstrate using lithographic the well known etching and depositiontechniques commonly used in semiconductor fabrication. Integratedcircuit (IC) dies are electrically connected to printed circuit boardsby wire bonds. The wire bonds are very thin wires—around 25 to 40microns in diameter—extending from contact pads along the side of thewafer substrate to contacts on the printed circuit board (PCB). Wirebonding is a widely used electrical interconnection technique because ofthe speed and accuracy of modern wire bonding machines, commonlyreferred to as wirebonders.

Wirebonders are automated devices that weld small lengths of wire fromconductors on the PCB to the contact pads on an integrated circuit die.Wire is fed through a bonding tool that uses some combination ofpressure, heat and/or ultra-sonic energy to attach the wire to the bondpads via a solid phase welding process. The two most common types ofwire bonder are referred to as wedge bond and ball bond. These refer tothe bonding tool and the configuration of the wire bond itself. Withboth types of wirebonders, the individual wire bonds extend in an arcfrom the bond pad on the integrated circuit (IC) die to the conductor onthe PCB. This is because wires from the contact pads to the PCB are madelonger than necessary to accommodate changes in the gap between the PCBand the bonds pads due to thermal expansion, flex in the components andso on.

To protect and strengthen the wire bonds, they are sealed within a beadof epoxy called encapsulant. The top of the wire arc is often about 300microns above the contact pads although some wire bonding may extendeven higher. As the name suggests, the encapsulant needs to encapsulatethe full length of the wire so the encapsulant bead will extend 500microns to 600 microns proud of the contact pads.

If the die is purely an electronic microprocessor, there is little needto keep close control of the encapsulant bead dimensions. However, ifthe die is a micro-electro mechanical systems (MEMS) device with anactive upper surface, it may be necessary or desirable to bring theactive surface of the die onto close proximity with another surface. Onesuch situation applies to inkjet printheads. The proximity of the printmedia to the nozzle array influences the print quality. Similarly, if acleaning surface is wiped across the nozzles, the bead of encapsulantcan hamper the wiping contact.

Another problems arises because of sides of the encapsulant bead are notstraight. One commonly used technique for depositing the encapsulantinvolves extruding it from a needle directly onto the line of wirebonds. The encapsulant volume and placement on the die is not veryaccurate. Variations in the pressure from the pump or slightnon-uniformities in the speed of the needle cause the side of the beadcontacting the active surface to be reasonably crooked. As the side ofthe bead is not straight, it has to be generously spaced from any activeparts on the active surface to comfortably accommodate theperturbations. Spacing the electrical contacts away from the activeportions (say for example, inkjet nozzles) of the active surface uses upvaluable wafer real estate and reduces the number of dies that can befabricated from a wafer disc.

In light of the widespread use of inkjet printheads, the invention willbe described with specific reference to its application in this field.However, the ordinary worker will appreciate that this is purelyillustrative and the invention is equally applicable to other integratedcircuits wire bonded to a PCB or other support structure.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a method ofprofiling a series of wire bonds between a line of contact pads on adie, and a corresponding set of conductors on a supporting structure,the method comprising the steps of:

electrically connecting each of the contact pads on the die to acorresponding conductor on the supporting structure with a respectivewire bond, each of the wire bonds extending in an arc from the contactpad to the conductor; and,

pushing on each of the wire bonds individually to collapse the arc andplastically deform the wire bond such that the plastic deformationmaintains the wire bond in a flatter profile shape.

The strength of the wire bond is known to be relatively small; of theorder of 3 to 5 grams force. However, the Applicant's work has foundthat the wire bond structure is robust enough to withstand a certaindegree of work hardening from plastic deformation. The arc of the wirebond can be deformed into a flatter profile without compromising theelectrical connection with the PCB. The Applicant's above referencedU.S. Ser. No. 11/860,539 (Our Docket MPN008US) discloses a technique forsimultaneously pushing some or all of the wire loops in the line of wirebonds. This so called gang wire pushing technique is effective butfurther development has shown that individually collapsing each wirebond is more controlled and easier to implement in a high volumemanufacturing process.

Preferably, adjacent wire bonds in the line of wire bonds aresequentially pushed. In a further preferred form, the step of formingthe line of wire bonds uses a wirebonder that has a bonding tool formoving between the contacts pads and their respective correspondingconductors, and the line of wires bonds are sequentially pushed by awire engaging structure on the wirebonder. Preferably, the wire engagingstructure and the bonding tools are configured for synchronizedmovement. Preferably, the wire engaging structure pushes the wire bondimmediately adjacent the wire bond currently being formed by the bondingtool. Preferably, the wirebonder is a wedge type wirebonder and thebonding tool is a wedge with a wire clamp at a distal end such thatduring use, the wire clamp holds a piece of wire in contact with one ofthe contact pads on the die to form a weld connection before moving tothe corresponding conductor on the PCB to weld the other end of the wireand formal wire bond, and the wire engaging structure has a wire pushingsurface that contacts the wire bond, the wire pushing surface isadjacent to, and 1.0 mm to 1.6 mm behind the wire clamp with respect tothe movement of the wedge towards the IC die. In a particularlypreferred form, the wire pushing surface is between 50 microns to 400microns closer to the PCB and the wire clamp. In some embodiments, theline of wire bonds does not extend more than 150 microns above thecontact pads of the IC die. In preferred embodiments, the line of wirebonds does not extend more than 50 microns above the contact pads of theIC die. In a particularly preferred form, the wire bonds are attached tothe contact pads with a bond strength greater than 3 g force.

Preferably the contact pads are spaced from the corresponding conductorson the PCB by more than 1 mm. In a further preferred form, the contactpads are between 2 mm and 3 mm from the corresponding conductors on thePCB. In some embodiments, the PCB has a support structure and the flexPCB and need to the support structure such that the conductors areadjacent the contact pads on the die. In particular embodiments, thesupport structure has a chip mounting area for supporting the die, thedie having a back surface in contact with the chip mounting area and anactive surface opposing the back surface, the active surface having thecontact pads, and the chip mounting area being raised relative to theremainder of the support structure such that the contact pads are raisedrelative to the conductors. In a particularly preferred form, thesupport structure is a liquid crystal polymer (LCP) molding. Preferably,the active surface has functional elements spaced less than 260 micronsfrom the contacts pads of the die. In a particularly preferred form, thedie is an inkjet printhead IC and the functional elements are nozzlesthrough which ink is ejected. In some embodiments, the support structureis a liquid crystal polymer (LCP) molding.

Preferably, the wire bonds are covered in a bead of encapsulant, thebead of encapsulant extending less than 200 microns above the activesurface of the die.

Preferably, the wire bonds are covered in a bead of encapsulant, thebead of encapsulant having a profiled surface that is flat, parallel toand spaced less than 100 microns from the active surface.

Preferably, the line of wire bonds are covered in a bead of encapsulant,the bead of encapsulant having a profiled surface that is flat andinclined relative to the active surface.

Preferably, the wire bonds are covered in a bead of encapsulant, theencapsulant being an epoxy material that is thixotropic when uncured.

Preferably, the wire bonds are covered in a bead of encapsulant, theencapsulant being an epoxy material has a viscosity greater than 700 cpwhen uncured.

In a particular embodiment, the printhead IC is mounted in a printersuch that during use the nozzles are less than 100 microns from thepaper path.

According to a second aspect, the present invention provides awirebonder for electrically connecting an integrated circuit die withconductors on a printed circuit board, the wirebonder comprising:

a bonding tool for attaching wire bonds from the integrated circuit dieto the conductors of the printed circuit board; and,

a wire engaging structure for deforming the wire bonds.

Preferably, the wire engaging structure pushes on the wire bonds toplastically deform them.

Preferably, the wire engaging structure is configured to push the wirebonds onto an adhesive surface positioned between the integrated circuitand the conductors of the printed circuit board.

Preferably, the wire engaging structure is flexible relative to thebonding tool.

Preferably, the wire engaging structure is configured for synchronizedmovement with the bonding tool.

Preferably, the bonding tool moves from the integrated circuit to theconductors when forming one of the wire bonds and the wire engagingstructure has a wire pushing surface positioned 1.0 mm to 1.6 mm behindthe bonding tool with respect to its direction of movement when formingthe wire bonds.

Preferably, the integrated circuit die is mounted to a supportingsurface and the wire pushing surface is 50 microns to 400 microns closerto the supporting surface than the bonding tool. Preferably, thewirebonder is a wedge type wirebonder and the bonding tool is a wedgewith a wire clamp at a distal end such that during use, the wire clampholds a piece of wire in contact with one of the contact pads on the dieto form a weld connection before moving to the corresponding conductoron the PCB to weld the other end of the wire and formal wire bond.Preferably, the wire pushing surface is formed from a material that hasa hardness less than that of the wire bonds.

Preferably, the wire bonds are formed from lengths of wire with a gaugebetween 15 microns and 75 microns. In a particularly preferred form, thegauge is about 25 microns.

In some embodiments, the wire bonds attach to respective contact pads onthe IC die and the wire bonds do not extend more than 150 microns abovethe contact pads of the IC die. In preferred embodiments, the line ofwire bonds does not extend more than 50 microns above the contact padsof the IC die.

Preferably the contact pads are spaced from the corresponding conductorson the PCB by more than 1 mm. In a further preferred form, the contactpads are between 2 mm and 3 mm from the corresponding conductors on thePCB. In some embodiments, the PCB has a support structure and the flexPCB and need to the support structure such that the conductors areadjacent the contact pads on the die. In particular embodiments, thesupport structure has a chip mounting area for supporting the die, thedie having a back surface in contact with the chip mounting area and anactive surface opposing the back surface, the active surface having thecontact pads, and the chip mounting area being raised relative to theremainder of the support structure such that the contact pads are raisedrelative to the conductors. In a particularly preferred form, thesupport structure is a liquid crystal polymer (LCP) molding. Preferably,the active surface has functional elements spaced less than 260 micronsfrom the contacts pads of the die. In a particularly preferred form, thedie is an inkjet printhead IC and the functional elements are nozzlesthrough which ink is ejected. In some embodiments, the support structureis a liquid crystal polymer (LCP) molding.

According to a third aspect, the present invention provides anelectronic device comprising:

an integrated circuit die with a plurality of contacts pads;

a printed circuit board with a plurality of conductors corresponding toeach of the contact pads respectively;

wire bonds electrically connecting each of the contact pads to thecorresponding conductors; and,

an adhesive surface positioned between the contacts pads and thecorresponding conductors; wherein,

the wire bonds are secured to the adhesive surface.

According to a fourth aspect, the present invention provides a method ofreducing wire bond loop heights in wire bonds electrically connecting anintegrated circuit die with a contact pad to a printed circuit boardwith a conductor, the method comprising the steps of:

mounting the integrated circuit die such that the contact pad is spacedfrom the conductor;

positioning an adhesive surface between the contact pad and theconductor on the printed circuit board;

attaching wire to one of the contact pad or the conductor;

drawing the wire towards the other of the contact pad or the conductor;

allowing the wire to contact the adhesive surface; and,

attaching the wire to the other of the contact pad of the conductor toform a wire bond adhered to the adhesive surface and a pointintermediate its ends.

These aspects of the invention are placed on the realisation that wirebonds can be adhered to an underlying supporting structure withoutdetrimental effects to their bond strength or function. Adhering thewire bonds between their ends provides a reliable and control productionin the wire bond loop height. The resulting wire bond heights can besmaller than that achieved by inducing plastic deformation as theindividual wire is unable to spring back up when the wire engagingstructure disengages. It will be appreciated that plastically deforminga wire bond also involves initially elastically deforming the wire. Theelastic deformation is removed when the wire pushing structure isretracted.

The wire bond can be adhered while the bonds are being formed by thewirebonder without any modification to the bonding tool. The applicanthas found that wire bond is will typically allow the wire to touch thesurface between the die and the conductors on the printed circuit boardas the bond is being formed. Once the wire has been welded to thecontact pad on the die, the bonding tool draws is towards the conductorson the printed circuit board. As it is drawn across the gap between thedie and printed circuit board, the wire drapes downwardly and rests onthe underlying surface. The only once the bonding tool has welded theother end of the wire to the conductor, and the wire clamp immediatelybehind the bonding tool breaks off the feed wire by pulling untiltensile failure, does residual tension in the loop cause it to bowupwards. If the wire is brought down to touch an adhesive surface beforethe ultrasonic weld on the printed circuit board, it is not able to bowupwards.

Preferably, the wire bond is moved into contact with the adhesivesurface by a wirebonder is the wire bond is being formed.

Preferably, the adhesive surface is one side of a double-sided adhesivetape. Preferably, the integrated circuit die and the PCB are mounted toa supporting structure such that they are adjacent and spaced from eachother. Preferably, the PCB is a flexible PCB and the supportingstructure is a liquid crystal polymer (LCP) molding. Preferably, theintegrated circuit die is mounted to the supporting structure by a dieattach film, and the adhesive surface is provided by a portion of thedie attach film.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a common prior art technique forapplying a bead of encapsulant to wire bonds;

FIG. 2 is a schematic representation of a die mounted to a supportingstructure with a chip mounting area raised relative to the flex PCBmounting area;

FIGS. 3A, 3B and 3C are schematic representations of the encapsulantbead being profiled into a desired shape using a moveable blade;

FIGS. 4A to 4D are schematic representations of wire bonds beingprofiled by plastic deformation;

FIGS. 5A and 5B show the encapsulant bead height reductions forplastically deformed wire bonds;

FIGS. 6A to 6C show the encapsulant bead being applied to the wire bondsusing the profiling blade;

FIGS. 7A and 7B show the profiling blade being used to control theencapsulant bead front on the surface of the die;

FIG. 8 shows the wire pusher mounted to the wirebonder;

FIG. 9 shows the wire pusher and the wire bonder in profile; and,

FIG. 10 shows the wire bonds adhered to the LCP molding between the dieand the flex PCB.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a common technique used for applying a bead encapsulant towire bonds. A die 4 is mounted to a supporting structure 6 adjacent theedge of a flex PCB 8 (flexible printed circuit board). The die 4 has aline of contact pads 10 along one edge and the flex PCB 8 hascorresponding bond pads 12. Wire bonds 16 extend from the bond pads 10to the bonds pads 12. Power and data is transmitted to the die 4 viaconductive traces 14 in the flex PCB 8. This is a simplifiedrepresentation of the dies mounted within many electronic devices. Theprinthead IC dies mounted to the LCP (liquid crystal polymer) molding toreceive print data from an adjacent flex PCB, as described in U.S. Ser.No. 11/014769 (Our Docket RRC001US) incorporated herein by crossreference, is one example of this type of die mounting arrangement. Theordinary worker will appreciate that the die may also be mounteddirectly to a hard PCB with traces formed thereon.

The wire bonds 16 are covered in a bead on encapsulant 2 to protect andreinforce the bonds. The encapsulant 2 is dispensed from a dischargeneedle 18 directly onto the wire bonds 16. Often the encapsulant bead 2is three separate beads—two beads of so-called ‘dam’ encapsulant 20, andone bead of ‘fill’ encapsulant 22. The dam encapsulant 20 has a higherviscosity than the fill encapsulant 22, and serves to form a channel tohold the fill encapsulant bead. The height H of the bead 2 above the die4 is usually about 500-600 microns. In most electronic devices, thisdoes not pose a problem. However, if the die has an active surface thatneeds to operate in close proximity to another surface, this bead can bean obstruction.

Elevating the Die Relative to the Flex PCB

FIG. 2 shows a stepped support structure 6 that has raised the chipmounting area 26 relative to the PCB mounting area 24 (or at least thearea mounting the PCB bonds pads 12). With the die 4 on a raised chipmounting area 26, the arc of the wire bonds 16 are lower relative toactive surface 28 of the die 4. In fact, the end of the wire bond 16attached to the contact pad 10 can be the apex of the arc (bearing inmind that the wire bond arc is intended to accommodate some relativemovement of the die and PCB). When the wire bonds 16 are covered withencapsulant 2, the bead has a reduced height H above the active surface28 of the die 4. If the bead of encapsulant 2 uses two beads of damencapsulant 24 and a fill encapsulant 22, the positions, volumes andviscosities of the beads need to take the step into account. Beadheights less than 100 microns are easily achievable, and with additionalmeasures, such as wire arc collapsing and bead profiling (discussedbelow), bead height of less than 50 microns are possible.

With the die 4 raised above the flex PCB 8 by 410 microns, the height ofthe wire bonds 16 above the die is about 34 microns. With the die raised610 microns above the flex PCB, the wire bond height is around 20microns. Raising the die even further has shown little or no furtherreduction in wire bond height with a step of 710 microns having a wirebond height of around 20 microns.

Shaping the Encapsulant Bead with a Profiling Blade

FIGS. 3A to 3C show the encapsulant 2 being profiled with a profilingblade 30. The support structure 6 is again stepped to reduce the heightof the wire bonds 16 above the die 4. Before the epoxy encapsulant 2 hascured, the profiling blade 30 moves across the die 4 and wire bonds in apredetermined path. As shown in FIG. 3B, the blade 30 displaces the topof the bead 30 to its flex PCB side to form a flat top surface 32 thatis at a significantly reduced height H above the die 4.

The encapsulant bead 2 may be a plurality of separate beads as shown inFIGS. 1 and 2, or a single bead of one material. However, for closedimensional control of the profiled encapsulant, the encapsulantmaterials used should be thixotropic—that is, once deposited from thedischarge needle, or profiled by the blade 30, the material should notflow under its own weight, but rather hold its form until it cures. Thisrequires the epoxy to have an uncured viscosity greater than about 700cp. A suitable encapsulant is DYMAX 9001-E-v3.1 Chip Encapsulantproduced by Dymax Corporation with a viscosity of approximately 800 cpwhen uncured. The blade 30 may be ceramic (glass) or metal andpreferably about 200 microns thick.

It will be appreciated that the relative movement of the blade 30 andthe die 4 can be precisely controlled. This allows the height H to bedetermined by the tolerance of the wire bonding process. As long as H isgreater than the nominal height of the wire bond arc above the die, plusthe maximum tolerance, the encapsulant 2 will cover and protect the wirebonds 16. With this technique, the height H can be easily reduced from500-600 microns to less than 300 microns. If the heights of the wirebond arcs are also reduced, the height H of the encapsulant bead can beless than 100 microns. The Applicant uses this technique to profileencapsulant on printhead dies down to a height of 50 microns at itslowest point. As shown in FIG. 3C, the lowest point is at theencapsulant front and the blade 30 forms an inclined face 32 in the topof the bead 2. The inclined face is utilized by the printheadmaintenance system when cleaning the paper dust and dried ink from thenozzle face. This illustrates the technique's ability to not just reducethe height of the encapsulant bead, but to form a surface that canperform functions other than just encapsulate the wire bonds. The edgeprofile of the blade and the path of the blade relative to the die canbe configured to form a surface that has a multitude of shapes for avariety of purposes.

Plastic Deformation of the Wire Bond Arcs

FIGS. 4A to 4C show another technique for lowering the profile of wirebonds. FIG. 4A shows the die 4 connected to the flex PCB 8 via the wirebonds 16. While the stepped support structure 6 has lowered the heightof the wire bond arcs compared to a flat supporting structure, the wirebonds still have a natural tendency to bow upwards rather than downwardstowards the corner of the step. The wires 16 are typically about 32microns in diameter and have a pull force of about 3 to 5 grams force.The pull force is the tensile load necessary to break the connection tothe contact pad 10 or the bond pad 12. Given the fragility of thesestructures (one of the reasons encapsulant is applied), conventionalwisdom is to avoid any contact between the wire bond arcs and othersolid surfaces.

As shown in FIG. 4B, the arc of the wire bonds 16 can be collapsed by awire pusher 34. The wire pusher 34 displaces the wire bond 16 enough toelastically and plastically deform the arc. The Applicants have shownthat contact with the wire pusher 34 can cause localized work hardeningin the wire, but as long as the pushing force is not excessive, it doesnot break. The end of the wire pusher 34 is rounded to avoid stressconcentration points. The wire pusher may be a stylus for engagingsingle wire bonds or a blade that pushes on multiple wire bondssimultaneously.

Referring now to FIG. 4C, the wire pusher 34 is retracted and the wiresprings back toward its original shape to relieve the elasticdeformation. However, the plastic deformation remains and the wire bondheight above the die 4 is much reduced. Testing has shown that aninitial wire bond loop height of 200 microns can be reduced to about 35microns using this technique. Tests have also shown that the pullstrength of the plastically deformed wires remains at about 3 to 5 gramsforce.

The collapse of the wire bonds is uncontrolled and leaves the wire bondssomewhat randomly deformed. However, pushing the wire bonds closer tothe die provides more uniformly shaped collapsed wire bonds. TheApplicant's work has shown that engaging the wires about 200 to 300microns for the die provides the best results.

As shown in FIG. 4D, the die 4 and the flex PCB 8 are mounted to a flatsupport structure 6. As discussed above, this means the original loopheight of the wire bond arc is much higher—approximately 400 micronsabove the die 4. Consequently, the wire has more plastic deformationwhen the loop is collapsed by the wire pusher. Even so, the Applicantsresults show that the residual loop height after pushing is about 20-50microns.

FIGS. 5A and 5B show the collapsed wire bonds 16 covered with anencapsulant bead 2. Even without bead profiling prior to curing, theheight H of the bead above the die is much less than the bead necessaryto encapsulate the original undeformed wire loops.

Applying Encapsulant with Profiling Blade

FIGS. 6A, 6B and 6C show the application of the encapsulant bead usingthe profiling blade 30 instead of a discharge needle (see FIGS. 1 and2). As previously discussed, the flowrate of encapsulant from thedischarge needle can vary and this gives rise to large variations on theposition of the encapsulant front on the active surface of the die 4.Consequently, any functional elements in the active surface of the dieneed to be sufficiently spaced from the contacts pads 10 to allow forthe meandering encapsulant front.

Applying the encapsulant with the profiling blade avoids the problemscaused by the flowrate fluctuations from the discharge needle. As shownin FIG. 6A, the bead of encapsulant 40 can be formed on the profilingblade 30 by simply dipping it into a reservoir of uncured encapsulantepoxy. Of course, the bead 40 may also be formed by any other convenientmethod, such as running the discharge needle along one end of the blade30.

FIG. 6B show the blade 30 having been lowered to touch the bead 40 ontothe die 4. When the encapsulant material touches the die surface, itwets and wicks along the surface while remaining pinned to the edge ofthe blade. The blade 30 is held at a predetermined height above the die4 and moved over the bead 2 to flatten and lower its profile. Theencapsulant displaced from the top of the bead 2 by the blade 30,spreads over the PCB side of the bead 2. It is not relevant if theencapsulant spreads further over the PCB than necessary. As long as thewire bonds 16 and the bonds pads 12 are covered, any additionalencapsulant on the PCB 8 surface is not detrimental.

In FIG. 6C, the wire bond 16 height has been reduced by collapsing thearc in accordance with the techniques discussed above. As previouslydiscussed, the bead 2 deposited by the discharge needle need not be asbig to cover the wire bond 16 once it has been collapsed. Furthermore,the blade 30 can be brought closer to the die 4 without contacting wirebonds 16 when profiling the encapsulant 2. Hence the bead profile inFIG. 6C is substantially lower than that of FIG. 6B.

Encapsulant Front Control

When the encapsulant material is dispensed from the discharge needle,minor variations in the flowrate can cause the bead to bulge at pointsof higher flow. Consequently, the side of the bead that contacts theactive surface of the die is not straight, but has significantperturbations. These perturbations have to be accommodated between thecontact pads and any functional elements on the active surface. Thespacing between the contacts pads and the functional elements consumesvaluable ‘chip real estate’. The Applicant has previously developedprinthead dies with a spacing of 260 microns between the contact padsand the first row of nozzles. Better control of the encapsulant frontreduces the space between the contacts and operational elements, and sothe overall dimensions of the die. Hence the design can be more compactand more chips fabricated from the original wafer disc.

As shown in FIGS. 7A and 7B, the profiling blade 30 is used to controlthe front 36 of the bead of encapsulant 2. The blade 30 is positionedover the die 4 to define a gap 42 between its lower edge and the activesurface 28. As the discharge needle 18 dispenses the encapsulantmaterial 44, it flows onto the active surface, one side of the blade anda fillet of the material extends through the gap 42. Because of the flowrestriction created by the gap, flow variations have a reduced effect onthe dimensions of the fillet that flows through the gap. Therefore theencapsulant front 36 closely corresponds to the line of the lower edgeof the blade 30.

As shown in FIG. 7B, the profiling blade 30 is already in position toprofile the encapsulant bead 2 once it has been dispensed from thedischarge needle. The blade 30 simply moves over the die 4 in adirection away from the nozzles 38. This keeps the encapsulant front 36in place and flattens the profile of the encapsulant bead 2 over thewire bonds 16.

Collapsing the Wire Bonds Arcs with the Wirebonder

FIGS. 8 and 9 show a technique for individually deforming each of thewire bond arcs using the wirebonder. This has several advantages overthe technique shown in FIGS. 4A to 4C described in the “PLASTICDEFORMATION OF THE WIRE BOND ARCS” section above. Firstly, deforming thewire bonds as they are being attached by the wirebonder is more timeefficient than pushing the line of wire bonds as a separate productionstep. Secondly, it has been found that individually engaging anddeforming each wire in the line of wire bonds provides uniform resultsin terms of wire deformation and bond strength.

FIG. 8 is a schematic perspective of a wirebonder 46 forming theindividual wire bonds 16 between the contact pads 10 and the conductors12 on the flex PCB 8. The printhead IC 4 is shown adhered to one side ofthe die attach film 58. In turn, the die attach film 58 is attached tothe LCP molding 6. Laser ablated holes through the die attach film 58feed ink to the array of nozzles 38. The LCP molding six has a steppedsurface 60 so that the printhead IC 4 is raised relative to the flex PCBeight. As discussed above this helps to reduce the height of the wirebonds 16.

The wire bonder 46 is commonly known in the industry as a “wedge type”wire bonder. The wedge 48 receives a stock of feed wire 56 at its tip.Using some combination of pressure, heat and ultrasonic energy, the endof the wire bond 16 is welded to one of the conductors 12 on the flexPCB 8, or one of the contact pads 10 on the printhead integrated circuit4. FIG. 8 includes an enlarged inset showing the connection between thewire bond 16 and the conductor 12. The end of the wire has a flattensection 54 created by the tip of wedge 48. Adjacent to flatten section54 is a neck portion 52 where the wire 16 transitions to its circularcross section. This neck section of the wire is work hardened andparticularly prone to plastic deformation. In light of the greaterpropensity for plastic deformation at the neck portion 52, the wirebonder 46 is fitted with a wire engaging structure 50 pushes on the wirebond 16 in this region. However, the wire engaging structure 50 to notcontact the wire bonds 16 too close to the neck portion 52 is this canbreak the wire. The skilled worker will appreciate the work hardeningincreases the brittleness of the metal. The applicant's testing hasfound that positioning the wire engaging structure 50 so that its tip isbetween 1.0 mm and 1.6 mm behind the tip of the wedge (with respect toits movement from the die to the flex) and 50 microns to 600 micronsbelow the tip of the wedge produces suitable results. In particular, thebest results were achieved with the tip of the wire engaging structure50 positioned 1.2 mm to 1.5 mm behind the tip the wedge, and 100 micronsto 300 microns below the tip of the wedge. This configuration gave wirebonds 16 less than 50 microns above the nozzle array 38, each with abond strength between 3.5 g and 5 g.

The wire engaging structure 50 is formed from a material with thesurface hardness less than that of the wire. This avoids surfaceindentations on the wire which may later become stress concentrationsites.

Adhering the Wire Bond to Reduce Loop Heights

FIG. 10 is a schematic section view of another technique used to reducethe height of the wire bond loops. An adhesive surface 62 is positionedon the LCP molding 6, between the contact pads 10 of the printhead IC 4and the corresponding conductor 12 on the flex PCB 8. The Applicant hasfound that a wirebonder will usually allow the wire bond 16 to drapedownwards and touch the surface between the die and PCB while the wireis bonded to the PCB conductor. Once the wire has been welded to thecontact pad on the die, the bonding tool draws it towards the conductorson the printed circuit board. As it is drawn across the gap between thedie and printed circuit board, the wire drapes downwardly and rests onthe underlying surface. Once the bonding tool has welded the other endof the wire to the conductor, and the wire clamp immediately behind thebonding tool breaks off the feed wire by pulling until tensile failure,the residual tension in the loop cause it to bow upwards. By positioningadhesive 62 at the point of contact between the wire and the LCP 6, thewire bond 16 is unable to bow upwards to the same height.

The adhesive surface 62 may be double sided tape, an adhesive paste orresin jetted onto the LCP 6 when the die 4 and the flex 8 are fixed, orit could simply be an extension of the die attach film 58.

The invention has been described herein by way of example only. Theordinary will readily recognize many variations and modifications whichdo not depart from the spirit and scope of the broad inventive concept.

1. A method of reducing wire bond loop heights in wire bondselectrically connecting an integrated circuit die with a contact pad toa printed circuit board with a conductor, the method comprising thesteps of: mounting the integrated circuit die such that the contact padis spaced from the conductor; positioning an adhesive surface betweenthe contact pad and the conductor on the printed circuit board;attaching wire to one of the contact pad or the conductor; drawing thewire towards the other of the contact pad or the conductor; allowing thewire to contact the adhesive surface; and, attaching the wire to theother of the contact pad of the conductor to form a wire bond adhered tothe adhesive surface and a point intermediate its ends.
 2. A method ofreducing wire bond loop heights according to claim 1 wherein the wirebond is moved into contact with the adhesive surface by a wirebonder isthe wire bond is being formed.
 3. A method of reducing wire bond loopheights according to claim 1 wherein the adhesive surface is one side ofa double-sided adhesive tape.
 4. A method of reducing wire bond loopheights according to claim 1 wherein the integrated circuit die and thePCB are mounted to a supporting structure such that they are adjacentand spaced from each other.
 5. A method of reducing wire bond loopheights according to claim 1 wherein the PCB is a flexible PCB and thesupporting structure is a liquid crystal polymer (LCP) molding.
 6. Amethod of reducing wire bond loop heights according to claim 1 whereinthe integrated circuit die is mounted to the supporting structure by adie attach film, and the adhesive surface is provided by a portion ofthe die attach film.
 7. A method of reducing wire bond loop heightsaccording to claim 1 wherein the wire bonds are formed from lengths ofwire with a gauge between 15 microns and 75 microns.
 8. A method ofreducing wire bond loop heights according to claim 7 wherein the gaugeis about 25 microns.
 9. A method of reducing wire bond loop heightsaccording to claim 7 wherein the wire bonds attach to respective contactpads on the IC die and the wire bonds do not extend more than 150microns above the contact pads of the IC die.
 10. A method of reducingwire bond loop heights according to claim 9 wherein the line of wirebonds does not extend more than 50 microns above the contact pads of theIC die.
 11. A method of reducing wire bond loop heights according toclaim 1 wherein the contact pads are spaced from the correspondingconductors on the PCB by more than 1 mm.
 12. A method of reducing wirebond loop heights according to claim 1 wherein the contact pads arebetween 2 mm and 3 mm from the corresponding conductors on the PCB. 13.A method of reducing wire bond loop heights according to claim 1 whereinthe PCB is a flexible PCB mounted to a support structure together withthe die such that the conductors are adjacent the contact pads on thedie.
 14. A method of reducing wire bond loop heights according to claim13 wherein the support structure has a chip mounting area for supportingthe die, the die having a back surface in contact with the chip mountingarea and an active surface opposing the back surface, the active surfacehaving the contact pads, and the chip mounting area being raisedrelative to the remainder of the support structure such that the contactpads are raised relative to the conductors.
 15. A method of reducingwire bond loop heights according to claim 17 wherein the wire bonds isformed using aluminium wire.
 16. A method of reducing wire bond loopheights according to claim 13 wherein the active surface has functionalelements spaced less than 260 microns from the contacts pads of the die.17. A method of reducing wire bond loop heights according to claim 16wherein the die is an inkjet printhead IC and the functional elementsare nozzles through which ink is ejected.
 18. A method of reducing wirebond loop heights according to claim 17 wherein the printhead IC isconfigured to be mounted in a printer such that during use the nozzlesare less than 100 microns from the paper path.